Toward single-layer Janus crystals: Off-balance materials from synthesis to nanotechnology applications

The existence of things is directly related to their structural symmetry in a broad framework ranging from atoms to crystalline materials and from simple cells to complex organisms like humans. However, structural imbalance that occurs through natural or artificial means can provide completely different advantages. Molecules, crystals, and complex structures with structural imbalance constitute the family of Janus-type materials. This perspective provides a comprehensive discussion on the synthesis techniques of Janus-type materials, their use in fields from biology to materials science, and very recent studies on the family of 2D ultrathin graphene-like structures. We believe that, thanks to the advances in experimental techniques, the few-atom-sized off-balanced materials will be indispensable parts of the nanotechnology products that soon will be used in our daily lives.